Pulp bleaching is usually carried out using hydrogen peroxide in an alkaline medium, since free-radicals are formed in an acidic medium at elevated temperature and these lead to undesirable secondary reactions such as degradation of cellulose. However, if a suitable catalyst is used, delignification and bleaching by means of hydrogen peroxide is also possible under acidic conditions.
U.S. Pat. No. 4,427,490 describes the delignification and bleaching of kraft pulp by means of hydrogen peroxide in an acidic medium, catalyzed by sodium tungstate or sodium molybdate.
In Journal of Pulp and Paper Science, Vol. 18, 1992, pages J108-J114, V. Kubelka describes a process for the delignification of pulp which has steps for delignification by means of oxygen and an intermediate delignification by means of hydrogen peroxide, which is carried out at a pH of 5 using sodium molybdate as catalyst. In the article, it is proposed that molybdate be recovered in a known manner using anion exchangers.
JP 11 130 762 describes the recovery of tungstate from a reaction mixture from the tungstate-catalyzed oxidation of maleic anhydride with aqueous hydrogen peroxide. The reaction mixture is for this purpose passed over a chelating resin having glucamine substituents and the resin is subsequently washed with aqueous sulphuric acid. The tungstate is then washed out of the chelating resin using aqueous sodium hydroxide. FR 2 320 946 describes the recovery of tungstate from the same reaction mixture with a strong-base anion-exchange resin.
JP 2003048716 describes a recovery of molybdate using a chelating ion-exchange resin.
CZ 279 703 describes a multistage recovery process for molybdate, in which molybdate is firstly adsorbed on a weak-base styrene-divinylbenzene ion exchanger and the molybdate is liberated in a subsequent step with aqueous ammonia.
JP 06010089 B proposes a chelating resin containing dithiocarboxyl groups for the recovery of molybdate.
However, the recovery of molybdate or tungstate using ion-exchange columns is too complicated and uneconomical for use in pulp bleaching because of the steps required for flushing of the ion exchangers. In addition, the degradation products of lignin formed in the delignification of pulp are adsorbed on the ion-exchange resins used in the prior art, which has an adverse effect on the ion-exchange properties of the ion-exchange resin.
In the proceedings of the 93rd annual conference of the PATPTAC 2007, pages A261-A268, R. C. Francis et al. propose precipitation of the molybdate catalyst in the catalyzed delignification of pulp by addition of the cationic surfactant cetyltrimethylammonium bromide and filtering off the complex formed. The molybdate is recovered from the filtered off complex by dissolution in sodium hydroxide solution and extraction of the cationic surfactant with a solvent such as isobutanol. However, the process proposed has the disadvantage that the complex formed from molybdate with cetyltrimethylammonium bromide is difficult to filter and the use of an additional organic solvent is necessary for recovery of molybdate.
For this reason, there continues to be a need for a process which is simple to carry out and whereby molybdate or tungstate can be recovered from an aqueous solution to a high degree, with the process being suitable for recovering molybdate or tungstate from the aqueous solutions obtained in the delignification of pulp.